Enzymes

There are a few things to remember about enzymes. First of
all, all enzymes are proteins (just about - there are a few
exceptions but you will not come across them in this class).
Also, don't get confused and think that all proteins are enzymes,
they aren't. You have already learned that the three dimensional
structure of a protein affects it function, so it should not be
surprise to you that those factors that denature proteins will
reduce the activity of enzymes. These include changes in pH,
temperature, and ionic strength.

The second thing to remember about enzymes is their function -
they are catalysts. Catalysts act to reduce the amount of
activation energy required for a reaction to occur. Many
reactions that are thermodynamically possible do not occur very
quickly (or seem not to occur at all) due to the requirement of
energy to get them started'. This required energy is called
activation energy. For example, a very common reaction in your
cells is the conversion of sugars such as glucose to carbon
dioxide and water. The delta G for this reaction is very negative
and so this reaction is exergonic and therefore a favorable one.
However, sugars such as glucose are very stable and can be stored
for years with no appreciable amount being converted to carbon
dioxide and water. This is due to the activation energy required
for this reaction. If you were to use a lit match to add energy
to some glucose, you would be providing the activation energy
required to start the reaction and glucose would quickly be
converted to carbon dioxide and water. This is what happens when
you use a match to light a piece of paper (remember, paper is
made of cellulose which is a polymer made from glucose). The
reduction in the amount of required activation energy allows for
the reaction to occur more quickly (without raising the
temperature) and thus increase the rate at which a reaction
occurs. So the take home message is that enzymes increase the
rate of a given reaction by reducing the need for activation
energy.

Properties of Enzymes

There are several general properties of enzymes that you
should remember when studying enzymes. Some of these are outline
below:

Denaturation: Since all (well, actually,
most) enzymes are proteins, their activity (ability to
function) is dependent on maintaining the proper three
dimensional conformation. Remember that the function of a
protein is dependent on its structure so that if the
structure is disrutped, so is its function. From your
knowledge of protein structures, you should remember that
changing factors such as pH, temperature and ionic
strength can lead to a disruption of the structure (and
thus the activity) of a protein. This disruption of
structure and function is known as denaturation. The
activity of enzymes therefore is usually altered by
changing the pH, temperature or ionic strength.

Specificity: Enzymes catalyze specific
reactions. This catalysis occurs in the a special site
where the reactants (known as substrates
in enzymatic reactions) bind and react to form the products.
This site is known as the active site of
the enzyme. The active site is specific for the
substrates so that almost no other substances will bind
there, and subsequently react. In a simplistic manner,
this is similar to the relationship of a lock and key.
Thus each enzyme will catalyze only one specific reaction
(there are a few exceptions, of course). This specificity
leads to another property - inhibition. Some substances
have similar structures to the true substrate of an
enzyme. An enzyme may not be able to distinguish between
the true substrate and the imposter. If the enzyme binds
the imposter at the active site, usually the enzyme can
not catalyze a reaction . This imposter molecule will
thus inhibit the activity of the enzyme. In such
conditions, the substrate and the imposter compete for
the active site and so such inhibition is known as
competitive inhibition.

Regulation: A cell does not always want
an enzyme to be active. For example, there is an enzyme
that synthesizes glycogen from glucose. If you were
starving, you would not want your cells to make glycogen
(remember glycogen is a storage form of glucose) - in
fact, you would want to break glycogen down to form
glucose. If you could not regulate the activity of
enzymes, you would not be able to prevent the cell from
making glycogen. For many enzymes, the cell has the
ability to turn up or down the activity of the enzyme
depending on the conditions in the cell and in the body.
Enzyme regulation generally occurs by one of two
different methods - allosteric regulation
and covalent modification.

In allosteric regulation, an enzyme has a second binding
site that is not catalytic (it is not an active site). This
second site, known as an allosteric site, is specific for a
particular molecule. When this molecule binds to the
allosteric site, the activity of the enzyme is altered.

In covalent modification, the enzyme structure is altered
by the addition of another compound what is attached to the
enzyme by a covalent bond. The most common modification
entails the addition of a phosphate (PO4) group.
Phosphate is negatively charged so the addition of such a
group alters the structure of the enzyme and thus its
activity. The addition of the phosphate group to the protein
(known as phsophorylation) is regulated as well (and so gets
very confusing - but that is for another course).